1. Field of the Invention
The present invention relates to a battery pack with a remaining battery power calculating function (hereinafter referred to as smart battery pack) which is reduced in cost and improved in performance.
2. Description of the Related Art
Smart battery packs of prior art use P-channel MOS transistors on the Hi side (see JP 2002-151163 A, pages 3 through 6, FIG. 1, for example). This can be seen in a circuit diagram of FIG. 5, which illustrates a conventional smart battery pack. The circuit in FIG. 5 has a plus side terminal 11, a minus side terminal 12, and terminals 13 and 14 for communicating with an electronic device. Although the battery pack in FIG. 5 has two communication terminals, only one communication terminal may be provided instead. The terminals 11 through 14 are connected to the electronic device or a charger. The smart battery pack contains a secondary battery 10, a protective circuit 1 for protecting the secondary battery 10, a circuit 2 for calculating the remaining capacity of the secondary battery 10, a resistor 3 for current detection, P-channel MOS transistors 4 and 5, and others.
The protective circuit 1 controls ON/OFF of the P-channel MOS transistors 4 and 5 in accordance with the state of the secondary battery 10. For instance, when the secondary battery 10 is in an over-discharged state, the protective circuit 1 turns the P-channel MOS transistor 4 off to prohibit discharge whereas the protective circuit 1 turns the P-channel MOS transistor 5 off to stop charging when the secondary battery 10 is in an overcharged state. In the case of lithium ion secondary batteries, a protective circuit is generally incorporated in a battery pack as a measure to enhance the safety level.
The circuit 2 for calculating the remaining capacity of the secondary battery 10 monitors the voltage of the secondary battery 10 as well as the electric potential on each end of the current detection resistor 3 to measure a charge current, a discharge current, and the like. The measurement results are transmitted to the electronic device through the communication terminals 13 and 14 (upon request of the electronic device).
The battery pack communicates with the electronic device in accordance with the GND reference, in other words, with the electric potential of the terminal 12 as the reference (hereinafter the GND reference means using the electric potential of the terminal 12 as the reference).
For the purpose of discussion, if an N-channel MOS transistor is used on the Lo side, the lower side electric potential of the secondary battery serves as the reference electric potential of the circuit for calculating the remaining capacity of the secondary battery of the battery pack, and does not match the GND reference electric potential of the electronic device. Accordingly, the battery pack and the electronic device cannot communicate with each other.
When the conventional smart battery pack communicates with the electronic device using the lower electric potential of the electronic device as the reference, a Hi side transistor of the battery pack has to be a P-channel MOS transistor. This is a problem since P-channel transistors in general have poorer mobility and characteristics compared to N-channel MOS transistors.